Seismic structure, gas hydrate, and slumping studies on the Northern Cascadia margin using multiple migration and full waveform inversion of OBS and MCS data

Abstract:

The primary focus of this thesis is to examine the detailed seismic structure of the
northern Cascadia margin, including the Cascadia basin, the deformation front and
the continental shelf. The results of this study are contributing towards understanding
sediment deformation and tectonics on this margin. They also have important
implications for exploration of hydrocarbons (oil and gas) and natural hazards (submarine landslides, earthquakes, tsunamis, and climate change).
The first part of this thesis focuses on the role of gas hydrate in slope failure observed
from multibeam bathymetry data on a frontal ridge near the deformation front
off Vancouver Island margin using active-source ocean bottom seismometer (OBS)
data collected in 2010. Volume estimates (∼ 0.33 km^3) of the slides observed on this
margin indicate that these are capable of generating large (∼ 1 − 2 m) tsunamis.
Velocity models from travel time inversion of wide angle reflections and refractions
recorded on OBSs and vertical incidence single channel seismic (SCS) data were used
to estimate gas hydrate concentrations using effective medium modeling. Results indicate a shallow high velocity hydrate layer with a velocity of 2.0 − 2.1 km/s that
corresponds to a hydrate concentration of 40% at a depth of 100 m, and a bottom
simulating reflector (BSR) at a depth of 265 − 275 m beneath the seafloor (mbsf).
These are comparable to drilling results on an adjacent frontal ridge. Margin perpendicular normal faults that extend down to BSR depth were also observed on SCS
and bathymetric data, two of which coincide with the sidewalls of the slump indicating
that the lateral extent of the slump is controlled by these faults. Analysis of
bathymetric data indicates, for the first time, that the glide plane occurs at the same
depth as the shallow high velocity layer (100±10 mbsf). In contrast, the glide plane
coincides with the depth of the BSR on an adjacent frontal ridge. In either case, our
results suggest that the contrast in sediments strengthened by hydrates and overlying
or underlying sediments where there is no hydrate is what causing the slope failure
on this margin.
The second part of this dissertation focuses on obtaining the detailed structure
of the Cascadia basin and frontal ridge region using mirror imaging of few widely
spaced OBS data. Using only a small airgun source (120 cu. in.), our results indicate
structures that were previously not observed on the northern Cascadia margin. Specifically, OBS migration results show dual-vergence structure, which could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Understanding the physical and mechanical properties of the basal layer has important implications for understanding earthquakes on this margin.
The OBS migrated image also clearly shows the continuity of reflectors which enabled
the identification of thrust faults, and also shows the top of the igneous oceanic crust
at 5−6 km beneath the seafloor, which were not possible to identify in single-channel
and low-fold multi-channel seismic (MCS) data.
The last part of this thesis focuses on obtaining detailed seismic structure of the
Vancouver Island continental shelf from MCS data using frequency domain viscoacoustic
full waveform inversion, which is first of its kind on this margin. Anelastic
velocity and attenuation models, derived in this study to subseafloor depths of ∼ 2
km, are useful in understanding the deformation within the Tofino basin sediments,
the nature of basement structures and their relationship with underlying accreted
terranes such as the Crescent and the Pacific Rim terranes. Specifically, our results
indicate a low-velocity zone (LVZ) with a contrast of 200 m/s within the Tofino basin
sediment section at a depth 600 − 1000 mbsf over a lateral distance of 10 km. This
LVZ is associated with high attenuation values (0.015 − 0.02) and could be a result
of over pressured sediments or lithology changes associated with a high porosity layer
in this potential hydrocarbon environment. Shallow high velocities of 4 − 5 km/s
are observed in the mid-shelf region at depths > 1.5 km, which is interpreted as
the shallowest occurrence of the Eocene volcanic Crescent terrane. The sediment
velocities sharply increase about 10 km west of Vancouver Island, which probably
corresponds to the underlying transition to the Mesozoic marine sedimentary Pacific
Rim terrane. High attenuation values of 0.03 − 0.06 are observed at depths > 1 km,
which probably corresponds to increased clay content and the presence of mineralized
fluids.

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